In the manufacturing processes of various semiconductor devices and TFTs (thin film transistors) of flat panel displays, a silicon nitride film is formed as, e.g., the gate insulating film of a transistor. As a method for forming a silicon nitride film, other than a method for depositing a silicon nitride film by CVD (Chemical Vapor Deposition), there has been proposed a method that forms a silicon oxynitride film by supplying nitrogen to a silicon oxide film by plasma processing, by, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2001-274148.
On the other hand, the film thickness of a gate insulating film is more and more decreasing as micropatterning of semiconductor devices advances. For example, demands have arisen for forming a thin gate insulating film having a film thickness of a few nm. It has become important to maintain a low leakage current and high reliability of a gate insulating film that is thus being more and more thinned. Presently, a silicon oxynitride film (SiON) is used as the material of a gate insulating film. As a method for further lowering the leakage current, however, it is being examined to use a high-k material as a gate insulating film. Also, a silicon nitride film having a higher dielectric constant (7.5) and a lower interface state density, as compared to those of a silicon oxide film, is obtained by directly nitriding silicon by use of plasma of a nitrogen-containing gas. Therefore, it is also being examined to form a silicon nitride film by directly nitriding silicon by use of plasma.
When forming a gate insulating film by directly nitriding silicon by the conventional plasma processing method, if the plasma nitriding process is performed at a temperature of from room temperature to 400° C. (inclusive), good insulating film characteristics become difficult to obtain as follows.
That is, in the actual transistor manufacturing process, processing (e.g., the formation of a poly-crystalline silicon electrode) at a high temperature exceeding 500° C. is performed after a gate insulating film is formed by low-temperature plasma processing. Therefore, the heat applied during this processing varies the insulating film characteristics of the gate insulating film. The present inventors have found that, where the plasma nitriding process is performed at a process temperature of, e.g., 400° C. or less, the amount of intermediate nitride (sub-nitride) existing in the interface (Si3N4/Si interface) between the silicon nitride film and silicon increases in the later annealing process, thereby varying the characteristics of the gate insulating film. This gate insulating film characteristic variation adversely affects the characteristics of a device such as a transistor, and deteriorates its performance.
Also, a silicon nitride film formed by directly nitriding silicon by plasma at a process temperature of 400° C. or less readily decreases the N-concentration (N-loss) or deteriorates the film quality by oxidation or the like with a lapse of time. In particular, the film quality deteriorates as the film thickness decreases. This makes stable silicon nitride film formation difficult.